Field of Invention
The present invention relates to a urea backflow prevention apparatus of selective catalyst reduction (SCR) and a control method thereof, and more particularly, to a urea backflow prevention apparatus of SCR and a control method thereof capable of preventing a supply pump from being damaged due to freezing of a backflowing urea solution.
Description of Related Art
An internal combustion engine vehicle using fossil fuel such as gasoline and diesel vehicles has a serious problem of environmental pollution due to exhaust gas. In particular, in the case of a diesel vehicle such as a bus and a truck, emission of smoke, nitrogen oxide (NOx), and fine dust including smoke has been recognized as a serious problem. To solve a problem of exhaust gas of the diesel vehicle, each country prepares related regulations to strictly regulate emission of exhaust gas.
Generally, an exhaust system of a diesel engine includes exhaust gas post-treating apparatuses such as a diesel oxidation catalyst (DOC), a diesel particulate matter filter (DPF), a selective catalyst reduction (SCR), and a lean NOx trap (LNT) so as to reduce carbon monoxide (CO), hydrocarbon (HC), particulate matter, nitrogen oxide (NOx), and the like which are pollutants included in exhaust gas.
Among those, the exhaust gas post-treating apparatus using the SCR (hereinafter, referred to as ‘SCR system’) serves to inject a reducer such as urea into an exhaust pipe to reduce nitrogen oxide in exhaust gas into nitrogen and oxygen. That is, when the reducer is injected into the exhaust pipe, the SCR system may convert the reducer into ammonia NH3 due to heat from exhaust gas and reduce nitrogen oxide into nitrogen gas N2 and water H2O by a catalytic reaction between the nitrogen oxide and the ammonia in exhaust gas using an SCR catalyst.
FIG. 1A and FIG. 1B are diagrams for describing an SCR system in accordance with the related art and FIG. 2A and FIG. 2B are diagrams for describing a problem of the SCR system in accordance with the related art. Referring to FIGS. 1A, 1B, 2A and 2B, when the SCR system is normally operated, the SCR system in accordance with the related art presses a urea solution stored in a urea tank by a pump so as to inject the urea solution into an exhaust pipe through a nozzle (see FIG. 1A). Further, when an engine stops, the urea solution is again recovered to the urea tank (see FIG. 1B). The purpose of recovering the urea solution is to prevent parts of the SCR system from being damaged due to an increase in volume of the urea which is caused by freezing of the urea when temperature becomes very cold during the winter.
However, after all the urea solution is recovered, when a vehicle stops or stays outdoor for a long period of time during the winter, an internal temperature of the SCR system in accordance with the related art balances outdoor temperature and then drops from a normal operation temperature (about 40° C.) below zero. In this case, a pressure of an internal channel of the closed SCR system which balances an atmospheric pressure is reduced at the time of recovering the urea solution (see FIG. 2A), such that the urea solution in the urea tank backflows to a supply pump through a suction line (see FIG. 2B). Therefore, when the backflowing urea solution is frozen, the supply pump may be damaged.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.